


Underfell Nightmare

by BlakeAster



Series: Underfell [1]
Category: Undertale (Video Game)
Genre: Alternate Universe - Underfell, I sort of hate myself for writing this, M/M, Mental Breakdown, Mental Instability, Miscarriage, Mpreg, SAAAAAANNNSSSSIIIIIMMMMMSSSSOOORRRYYY, This is going to be messed up, Underfell Asgore, Underfell Papyrus, Underfell Sans, Underfell W. D. Gaster, Yaaaaaayyy, my first fic!, oh god what am i DOING, pedo gaster, this is one of those that'll make you hate gaster
Language: English
Status: Completed
Published: 2016-10-01
Updated: 2016-10-05
Packaged: 2018-08-18 20:21:02
Rating: Mature
Warnings: Graphic Depictions Of Violence, Rape/Non-Con, Underage
Chapters: 6
Words: 3,475
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/8174789
Author URL: https://archiveofourown.org/users/BlakeAster/pseuds/BlakeAster
Summary: Pain was all he knew.If it wasn't the experiments, it was the beating.If it wasn't the beatings, it was...It was only a matter of time before he broke.





	1. Chapter 1

**Summary for the Chapter:**

> Hello! This is my first fic!   
> *sweats nervously  
> I may be shooting myself in the foot starting with something so dark, but I’m committed now. Please forgive any mistakes in regards to the AU as I was just recently introduced to Underfell. If you spot any grammatical mistakes or have any constructive criticism, I’d gladly appreciate it.   
> I wanted to say before hand that this is going to be a three part fic inspired by both Handplates and Flowerfell. That being said, I have never touched anything Flowerfell as I cannot handle sad stories and the short comic dub I stumbled upon is enough for me.   
> First chapter’s short. More of a prologue really, but I hope it’s enough to pique your interest, even a little.

Gaster felt nothing when his wife died.  
It wasn’t shock or anything like that. No, no. The woman had been nothing more than a means to an end. He’d needed children for his research. Unfortunately, he also needed children from his own particular breed of monster. Alas, he and his wife had been the only two left. It was the reason he had seduced her into marriage. He had hoped to get as many offspring as he could from her. He hadn’t expected her to die after a single pregnancy.  
Well, at least she had twins.   
The miserable things had cried quite a bit. He had them laid out before him on the operating table still covered in their mother’s dust. The larger and several minutes younger of the two was the loudest. The other was half his size and was doing little more than sniffling. Gaster pulled up the younger’s stats. He grinned at what he saw.   
“You have quite high base stats for a newborn. Yes… so much potential in this one. It might have taken…”  
It was the cause of their mother’s death after all. The experimental drug mixed with monster magic and determination. She’s never known of course. Gaster had known it was a risk, but if it worked, it was worth it. Part of him feared that the drug was going to kill the fetus as well, but he was out of options. The other experiments had suggested that older monsters couldn’t handle the determination, but the younger the subject, the longer they lasted.   
He hadn’t wiped out his own breed of monster to stop now.   
He switched to the smaller of the two and frowned.  
1 HP.  
1.  
Even the weakest of monsters started out with at least 10 HP.  
But 1?  
It was unheard of.  
It was pathetic.  
Obviously this one had been a failure.  
Ah, well, at least the other still had a shot. He would continue to administer the drug on a daily basis, slowly increasing the dosage. He’d be feeding them that instead of the usual magic that his breed fed their young. He considered disposing of the smaller one, but decided against it. He wasn’t going to get another chance, so he might as well continue with both. Besides, he might find some other use for the runt.


	2. Chapter 2

Pain was something both of the twins had grown used to. The experiments and tests their ‘father’ ran on them were painful and never ending. The elder one, Sans, had it the worst. Unlike his younger and stronger brother, his father didn’t particularly care if Sans didn’t survive one of his tests. Gaster sometimes wondered why he hadn’t already disposed of him. Though if he were to be perfectly honest, having him around did have it’s upsides.  
“Come on, you useless lump!”  
Sans glared up at his brother with fierce, red eyes. He charged at the other brother only to be thrown back onto the ground. Papyrus grinned sadistically at his ‘older’ brother. Gaster had noticed that the younger had grown to enjoy inflicting pain on the smaller child. More than likely, the larger was directing the anger and spite he had for Gaster on his brother, as he was the only available ‘punching bag.’  
Gaster found it interesting how it had escalated. It started with just a small argument on day only a few months ago. Another argument a week after that. Than a louder argument a few days later. Then came the first hit. It wasn’t much of a hit really. More like a slap. Papyrus had stared at his own hand like it had offended him somehow. Sans was just stunned. But Gaster had seen it. It was only a moment, but Gaster didn’t miss anything.   
Papyrus had enjoyed it.  
He’d enjoyed being the one inflicting pain on another for a change. It didn’t take too long after that for such displays of violence to become common among the brothers. Sans rarely got in a hit, but it didn’t stop him from trying. To begin with, Papyrus fought the smile that came to his lips. Now, he grinned. His teeth were bared like a wild animal.   
This one obviously took after his father in that way.  
It wasn’t until he had given them their first IQ test that he saw where Sans’ strength lie. The boy had finished the test almost as fast as Gaster had himself at his age. Obviously he wasn’t nearly at Gaster’s own level, but he was close.   
Honestly, Gaster would have been proud of his creations if the damned drug would just work.   
All these years and nothing. Not even from Papyrus, whom he had so much hopes in. Gaster growled. So much time and effort. So many resources had gone into this experiment and what did he have to show for it?   
“Hold still!” he snapped at Sans.  
Sans flinched, but did as he was told. He sighed in relief as his limb was reattached. That had been the first time Papyrus had managed to take one of his limbs off. It hurt. He didn’t like it at all. He rubbed his shoulder and prepared to jump off from the operating table when a large hand stopped him. He looked up warily at Gaster.   
Sans didn’t like the look in his eyes. He had never looked at either of them like that before. Something was wrong. He needed to get back to the cell. He wanted to be back with Papyrus, even if it meant losing the rest of his limbs.  
Gaster gave him a twisted smile.  
“I think I have a use for you.”

Xxx

“Stop your whining you big baby!” Papyrus snapped from his cot.   
Across the room, Sans was curled in a ball under his thin cover. He sobbed quietly as he shook. He couldn’t help but wonder what he had done to deserve all of this. Any of this. He hadn’t laid a finger on anyone except to defend himself from Pap. What had he done wrong?  
He wanted to ask, but decided he was too afraid the answer.


	3. Chapter 3

Papyrus didn’t like it.  
He didn’t like it at all.  
He knew what his father had been doing to his brother, and it pissed him off. It wasn’t out of disgust or some sort of familial feelings he held for his brother. Not that there was much of that left at this point. He couldn’t describe it. No wait, he did. It was like that one toy Gaster had given him. Well, technically it was for both of them, but Papyrus claimed it as his so it was. Sans was his, just as that toy had been his. Not that he wanted to touch his brother in such a manner. Found it quite repulsive actually. But he was his brother. Sans was his. He didn’t like anyone touching what was his, no matter how little it was.  
Gaster had left Papyrus in the room that time. Papyrus watched in disgust. He knew he couldn’t leave. Gaster would never had let that pass. He could look away, but that was almost worse. The rage was burning inside him. This man was claiming what was his and had the gall to do it right in front of him. And he knew he was doing it on purpose. He wanted to see how he would react. What he would do.   
Sans was sobbing again on the table while Gaster cleaned himself up. He never bothered to do the same for Sans. Gaster was watching Papyrus out of the corner of his eye. Calculating. Papyrus growled. The smirk that crept upon the scientist’s lips only made Papyrus even angrier. Sans was still crying. God, why couldn’t he just shut up!  
“Papyrus, would you mind taking your brother back to your room?” Gaster asked.   
Papyrus huffed but did as he was told. He was none to gentle as he grabbed Sans’ arm and yanked him off the table. The naked boy landed on the ground with a shout. Papyrus snarled.  
“GET UP YOU LAZY LUMP!” Papyrus snapped.  
Sans had only just managed to struggle to his feet when Papyrus began to drag him towards their room. Sans stumbled and limped behind him. Papyrus slammed the door to their room open and threw Sans onto his cot. Papyrus slammed the door shut again and walked over to their dresser. Inside the dresser was nothing but the medical gowns they had grown used to. He took one out and flung it at his brother.   
“GET DRESSED AND CLEAN YOURSELF UP. I’D RATHER NOT HAVE THAT MAN’S STENCH CONTAMINATING MY FOR ANY LONGER THAN NECESSARY,” Papyrus said.  
Sans didn’t move. For a moment, he was tempted to run back over and beat whatever tears he had left out of him. But he stopped. He boiled with anger and rage. Usually he would just take his anger out on the other boy, yer he couldn’t bring himself to do it. He rolled his eyes and plopped himself down on his own cot. He rolled so he was facing away from his brother. Despite his anger, the experiments had drained him pretty badly and it didn’t take long for him to fall asleep.   
If he had been awake just a minute longer, he would have heard the soft, almost hysterical laughter from across the room.


	4. Chapter 4

**Notes for the Chapter:**

> You may or may not have noticed that I keep updating the tags. I keep straying from my original outline and adding things.  
> Speaking of which, I've finished writing this story so it should be completed by Wednesday so long as I have access to a computer to update it, after which I'll begin work on the next part.  
> Thank you all for your support with this story. I was kinda nervous about posting it.

Gaster was beginning to think that this whole thing had been a grave mistake. He had wiped out an entire breed of monsters and what did he have to show for it? They were both failures. At least he could still use the ‘boys’ for breeding when they were mature enough. The fact of the matter was, their particular breed of monsters did not have genders. Not really. Sure, they usually chose an ‘identity’ based on how masculine or feminine they felt they were, but all of them were capable of both being a sire and carrying a child. So at least he could use them as breeders.He preferred the elder, but the younger may be better for those purposes.  
Gaster picked at the new crack on his skull, courtesy of the King himself.  
“Sans,” he called calmly as he entered the boys’ room. Sans was on his cot, reading a book he had taken from Gaster’s library. He tried to hide it, but Gaster snatched it out of his hands. Gaster smirked at him.  
“Now, what have I told you about this?” he said with a sickeningly sweet voice.   
He didn’t give him a chance to reply before grabbing his wrist to drag him off his cot. He didn’t get far before bone surrounded by bright red magic came flying at him. Gaster didn’t even seem to react, but a Gaster Blaster blasted the bone into oblivion. Gaster peaked over to Papyrus who was standing in an offensive position with murder in his eyes.   
“What do we have here?” Gaster asked rhetorically.  
Papyrus sent another barrage of attacks, which Gaster easily blocked.  
“YOU KEEP YOUR HANDS OF MY BROTHER!” Papyrus growled.  
“Oh? Yours, is he?” Gaster chuckled.  
“YES! HE’S MY BROTHER! H E ‘ S M I N E.”  
“Oh, on the contrary, you both belong to me,” Gaster said coolly.   
Sans watched Gaster beat his brother down without lifting a finger. He sent one magic attack after another. He didn’t stop until Papyrus was on the verge of turning into dust. One more hit, and would have been over. Gaster smirked.   
“I’ll stop here for now, but if you need any further lessons, I’m sure that Sans would enjoy a… break.”  
Papyrus flinched.  
Gaster turned back to Sans. Sans stared in shock at his brother. Gaster didn’t like this room, and those cots were far from comfortable.  
But it would do for today.

Xxx

Papyrus whimpered as Sans used what little knowledge he had of healing magic to heal his brother’s wounds. He had learned it from the very book he had been reading when Gaster walked earlier. Sans didn’t respond. Papyrus didn’t push. Soon, Papyrus fell asleep. Once he had done what he could, Sans laid his brother onto his cot and tucked him in. Sans limped across the room, but stopped when something caught his eye.  
There was a single mirror in the room. He saw what he’s usually expect. His own, tired, ugly mug and the small cracks and scars that littered his body where it was visible under the medical gown.   
But he had never seen the girl before.   
It was a human, he knew. She was maybe only a little older than himself, though smaller than Papyrus. She had brown hair and tanned skin. She was dressed in a black sweater with red stripes over jean shorts and black boots. Sans whirled around.  
No one was there.  
Sans blinked in confusion. He looked back to the mirror, but the Human was gone.


	5. Chapter 5

The mirror was only the beginning.  
Sans saw the girl everywhere he went. On every reflective surface he came across. She always had that same soft smile that made his bones tingle. He even saw her out of the corner of his eye sometimes, though she was always gone by the time he looked. He didn’t mind really. He had taken to staring into the mirror for hours on end, just watching her. He refused to blink, because he knew if he did she would vanish. Sans sat cross legged in front of the mirror. In the mirror, she was sitting beside him. She blinked lazily at him, but made no other movement.  
She was beautiful and unnerving at the same time.   
Papyrus was usually the one who broke him out of his daze. He had noticed how much Sans had been staring at mirrors recently and used it as another reason to beat him. Honestly, Sans has no idea how he hadn’t died yet. Both Papyrus and Gaster had somehow managed to make him suffer as much as possible without ever actually causing and real damage to his HP.  
He wished they wouldn’t bother.  
Still, every chance he got, he watched her. She was so pretty. She seemed to glow in the gray world he had grown accustomed to. The only light in his life.

Xxx

Gaster was amused by the most recent development.   
It started the other night. Honestly, Gaster was almost surprised when he saw the change in Sans while in the middle of one of their… liaisons. At first Gaster thought he was imagining it. He chuckled when he realized he wasn’t.  
It looked like his little boy was starting to… enjoy himself.   
After he’s finished, the twisted smirk told Sans that Gaster had noticed. Sans seemed so shocked and horrified with himself. He was almost completely unresponsive as he was dragged back to his room and thrown unceremoniously onto his cot. Papyrus grumbled something after Gaster has closed the door. Papyrus spared Sans a quick glance before turning over to go back to sleep. Not like there was anything else to do.  
At first he didn’t register the sound. When he did, he wasn't quite sure he was actually hearing what he was hearing. Papyrus sat up and looked over to his brother’s side of the room in confusion. Sans was facing away from him, curled up in the fetal position. His sides were shaking as though he were crying, but the sounds coming from him had nothing to do with crying.  
He was… laughing.  
The word seemed wrong however. There was nothing joyful about the sound. It was cold and disturbing. It was hysterical and growing louder and more disturbing by the second.   
“S-SHUT UP,” Papyrus demanded. His voice was shaky and betrayed his fear. “SHUT UP!”  
He didn’t. Papyrus stood and ran over to the other side of the room. He grabbed Sans by the back of his gown and pulled front the bed and onto the floor. He kicked. He kicked and he kicked but he didn’t stop. Sans just kept laughing. If anything, the laughter became more deranged.  
“SHUTUPSHUTUPSHUTUPSHUTUPSHUTUPSHUTUP!”  
Papyrus screamed in frustration. He went back to his bed and pulled his pillow over his head. Sans continued to laugh. He rolled over to face the mirror in the room. There she was, as beautiful as ever and with that same smile on her face. Only this time it was different. Her smile was sad and tears streaked down her face.   
His laughter began to fade, but he didn’t stop. It became breathless and stuttering as his tears joined hers.   
“Haha… ha…. Hahaha… haha...ha…”  
“...ha…”


	6. Chapter 6

**Notes for the Chapter:**

> This is the last chapter of the first part! Woohoo!  
> I'm already working on the next part. I'm not gonna start posting until I have a large portion of it done, however. Keep an eye out for it!

Gaster stared at the test results front Sans’ latest pregnancy. It had made it about 8 weeks. It was the fourth one since Sans and reached the maturity needed to begin breeding. It was infuriating. The fetus’ had shown a much better reaction to the determination drugs than either of the brothers, but none had survived. At first, he was worried that something was going wrong due to the determination, but know he knew that wasn’t the case. It wasn’t the drugs at all.  
It was Sans.  
Sans was feeding them too much magic. Not that he was doing so on purpose. It was beyond anyone’s control. The unborn monsters couldn’t handle it. Unfortunately, there was no way to fix the issue. Honestly, he had become quite fond of the smaller brother. He had wanted to use him to create more… test subjects, but it looked like that was off the table.  
“WHY AM I HERE?” Papyrus asked. “IF YOU AREN’T GOING TO START YOUR POKING AND PRODDING CAN I GO BACK PLEASE?”   
The ‘please’ was dripping with contempt. Gaster turned to face the younger boy.   
“Don’t be so impatient. We will begin now,” Gaster said.   
Gaster walked over to the operating table Papyrus was sitting on. He had already instructed the boy to remove his gown before sitting down. Papyrus was wary and uncomfortable, but he was trying his best to act nonchalant.   
His facade faded the moment Gaster began to unzip his pants.   
Papyrus bolted for the door. Gaster caught him by his arm and threw him back onto the table. Papyrus fought and clawed as Gaster loomed over him. Gaster frowned. He hadn’t had this much trouble in a while. Especially not front Sans. Since his breakdown, the boy and been more than compliant with Gaster’s will. He was quite the trained little boy now.   
Papyrus would get there eventually.   
Gaster was getting ready to spread the boy’s legs apart when it happened. The world turned red, than black. He was floating. He looked around in confusion. There was nothing but black. No, not nothing, Gaster corrected. He looked behind him to find he wasn’t alone.  
Sans stood there, his left eye glowing bright red.  
“What did you do?” Gaster growled.  
“Today already happened,” Sans said.  
Gaster blinked.  
“I think… it may have been a human. Determination. That was the power it gave them wasn’t it? The ability to manipulate time?” Sans asked.  
Gaster started to grin. If what he was saying was true then… than it had worked! After all this time, it had actually worked!  
“I saw today. Saw what you did to Papyrus after… He was pregnant you know. Gave birth to a little boy. It killed him though. Him and the child. I didn’t realize I could do this until after. Used it to trap you here,” Sans s aid.  
Gaster’s grin fell.  
“What are you-”  
If he had reacted, he might have escaped. If he had made a move, tried to do something, he might have not been trapped. But he couldn’t. The moment he saw the smile that broke across Sans’ face stopped him dead in his tracks. That toothy, twisted, sadistic grin made his soul shake.   
“Goodbye father.”

Xxx

Papyrus hadn’t moved front where he was when Gaster had suddenly disappeared from the room. He almost didn’t react when Sans seemed to materialize out of nowhere. He stared at him in shock. His grin took up half his face. It was almost as disturbing as his laugh the night he had laughed throughout the night.  
“Let’s go bro,” Sans said.  
“WHAT?”   
“Let’s ditch this joint,” Sans said.  
“WH-WHAT ABOUT FATHER?” Papyrus asked.  
“Oh,” Sans shrugged. “He’s a little lost. I don’t think he’ll be coming back.”  
“WHERE ARE WE GOING TO GO?”   
“Dunno,” Sans said. “Tibia-nest, I don’t really care.”  
“SANS… WHAT DID YOU JUST…”  
Sans wasn’t listening. He turned to the large mirror in the lab. He needed to see her. He was finally free, and he was scared. What were they going to do now? Where would they go? He was terrified, and he needed her. His light. Her smile. The only good thing in his life. His smile wavered.  
She was gone.


End file.
